Mingxia Cao

The transformation behavior and the shape memory effect due to cyclic stress/strain for Ti-49.6Ni alloy

Abstract The transformation behavior for Ti–49.6 at.%Ni shape memory alloy was investigated under cyclic stress/strain by electrical resistance method and differential scanning calorimetry (DSC) method. After solution no R transformation occurred during the martensite transformation in the R – T curve, and the reverse martensite transformation temperature points A s as well as A f could be discerned clearly in the heating process. With the same kind of heat treatment, R transformation occurred after the specimen underwent cyclic stress or strain; in the meantime, the reverse transformation temperature points A s and A f were illegible during the reverse martensite transformation. The characteristics of resistance–temperature curve ( R – T curve) of Ti–49.6Ni shape memory alloy changed apparently and regularly. The results also showed that after cyclic strain, the transformation temperatures M s and A s were stable while the cyclic stress is not. That implied that the low-strain cycling itself could be used as a better method for mechanical training. For both stress and train cycling, the martensite transformation temperature M s was lower than that of the raw material. It indicated that stress/strain cycling would inhibit the martensite transformation and directly result in the R transformation occurring. The shape memory effect after cyclic stress/strain were also studied, respectively.

Improved GaN film overgrown with a molybdenum nanoisland mask

We report the improved crystalline and optical quality of GaN film overgrown by hydride vapor phase epitaxy adopting a molybdenum (Mo) nanoisland mask (MNM). The MNM is fabricated following thermal annealing of the nanometer-thick Mo film deposited by electron-beam evaporation. The full width at half maximum values of high-resolution x-ray diffraction (HRXRD) rocking curves for the GaN film with MNM are 188arcsec (002 reflection) and 219arcsec (102 reflection), while those for the GaN film without MNM are 256 and 364arcsec, respectively. This result indicates a significant reduction of dislocation density in the overgrown GaN film with MNM. Photoluminescence spectra measurements reveal the compressive strain relaxation and improvement in the quality of the overgrown GaN film with MNM as compared to the regrown GaN film without MNM, which is consistent with the trend observed by HRXRD.

Effects of Cr and Sn on defects and electron densities in TiAl alloys

Positron lifetime spectra of Ti50Al50, Ti50Al48Cr2 and Ti50Al48Sn2 alloys have been analyzed. The experimental results show that the electron densities in bulk and grain boundary of binary TiAl alloy are lower than that of Pure Al and Ti metals. The addition of Cr atoms to TiAl results in the increase of the electron density in bulk and grain boundary of the alloy. Cr atom can provide more free electrons than Ti or Al atom to participate in metallic bonds in the alloy. However, the addition of Sn to TiAl alloy has the opposite effect in comparison with Cr addition. It is suggested that replacing the Al (or Ti) atoms in TiAl alloy by using metallic elements with a bulk positron lifetime lower than that of pure Al (or Ti) metal may increase the free electron density and produce a good effect on room temperature ductility and vice versa

Thick GaN grown on a nanoporous GaN template by hydride vapor phase epitaxy

High-quality thick gallium nitride (GaN) films were overgrown by hydride vapor phase epitaxy (HVPE) on a nanoporous GaN template which was prepared by inductively coupled plasma etching employing an anodized aluminum oxide mask. An obvious reduction of the dislocation density in the thick GaN layer was demonstrated by high-resolution X-ray diffraction, which exhibited the improved crystalline quality in GaN films overgrown on a nanopattemed surface. Moreover, the peak redshift in photoluminescence and micro-Raman spectroscopy indicates a significant strain relaxation in the HVPE-GaN layer. Compared with conventional overgrowth, such a deposition pathway is a more promising technique for the growth of thick GaN layers.

High-Quality Thick GaN Overgrown on an Array of SiO2 Nanomasks by HVPE

We report nanoepitaxy of thick GaN films by hydride vapor-phase epitaxy (HVPE) using a uniform array of SiO 2 nanomasks that were prepared by electron-beam evaporation and anodic aluminum oxide membranes. The controllable size and density of the SiO 2 nanoisland mask were about 65 nm and 10 10 cm -2 , respectively. Subsequent overgrowth takes place selectively on the exposed GaN surface without the SiO 2 masks to the formation of the continuous layer. High-resolution X-ray diffraction shows reduction of threading dislocations in the overgrown layers. Improved optical property and significant stress relaxation in the epilayer are demonstrated by photoluminescence and micro-Raman spectra. The deposition method is a more promising technique for obtaining freestanding GaN substrates with high-quality and uniform features.